In wireless communications, error control techniques for correcting errors not recovered with the use of an equalization technique or a diversity technique, etc., are popularly used for achieving a high-quality transmission. Among such error control techniques is an Automatic Repeat Request (ARQ: hereafter referred to as “ARQ”).
In the ARQ, in which a transmission side and a reception side is connected via a two-way transmission path, the transmission side sends a packet containing a codeword generated by performing an error detection encoding processing on information bits to the reception side, and the reception side then performs an error detection. In a case where no error is detected in the received data, the reception side returns a reception acknowledgement signal notifying that reception is done correctly (Positive Acknowledgement: hereafter referred to as “ACK”) , whereas in a case where an error is detected in the received data, the reception side returns a retransmission request signal (Negative Acknowledgement: hereafter referred to as “NACK”). Upon reception of a NACK, the transmission side retransmits the identical packet. Until receiving an ACK, the transmission side repeats retransmission of the identical packet.
An explanation is given here with an example of a case where information bits subjected to blocking processing are sequentially transmitted in a packetized composition. First of all, a transmission side transmits the 1st packet M to a reception side, and upon correct reception of a codeword contained in the 1st packet, the reception side sends an ACK to the transmission side. Receiving the ACK, the transmission side sends the next 2nd packet M+1 to the reception side. Next, receiving the 2nd packet M+1 incorrectly, the reception side sends a NACK to the transmission side. Receiving the NACK from the reception side, the transmission side resends the 2nd packet M+1 to the reception side (repeat transmission). That is, until receiving an ACK from the reception side, the transmission side continues to transmit the packet M+1, which is the same packet as one transmitted in the last attempt for transmission, without going on to send the next new 3rd packet M+2. In this manner, a high-quality transmission is achieved in the ARQ.
Though a high-quality transmission is achieved in the ARQ described above, this technique could sometime cause a transmission delay due to repetitive retransmission. Especially under a poor propagation environment, a high data error rate will inevitably increase the number of times of retransmissions, resulting in a sharp increase in a propagation delay. These days, active study efforts have been directed to a hybrid ARQ as a technique for dealing with a problem of the propagation delay in the ARQ. The hybrid ARQ is a scheme which incorporates an error correction coding technique into the ARQ, aiming for a decreased number of retransmission times and for a consequentially improved throughput, achieved by enhancing the error rate of a reception signal using an error correction technique.
Among hybrid ARQ schemes as described above is a Packet Combining type hybrid ARQ technique. According to the Packet Combining type hybrid ARQ, a transmission side retransmits a packet M, which is identical to its last transmission packet, that is, the packet M. Upon reception of the retransmitted packet M, a reception side performs a combination processing of a codeword (systematic bits and parity bits) contained in the packet M received in the last transmission and before the last combined with a codeword (systematic bits and parity bits) contained in the packet M received in the current transmission, and then performs an error correction decoding on the combined signal. In this way, according to the Packet Combining type hybrid ARQ, the reception level is enhanced by combining the codeword contained in the packet M received in the last transmission and before the last transmission with the codeword contained in the packet M received in the current transmission, and therefore, the error rate in a reception signal is improved as retransmission is repeated. This makes it possible to receive a reception signal without errors in a lesser number of retransmission attempts than in that of an ARQ without error correction, which makes it further possible to enhance throughput.
However, in the above-described hybrid ARQ technique, there may be a case where an ACK or a NACK reaches erroneously to the reception side due to degradation in its propagation environment, where in such a situation, the transmission side may transmits a packet which is not what the reception side demands. More specifically, when an error is detected in the packet M at the reception side, and accordingly, when a NACK message is transmitted to the transmission side, the transmission side may, notwithstanding the transmission of the NACK, recognize it erroneously as an ACK, where in such a case, the transmission side goes ahead to transmit the next packet M+1. Having requested retransmission, the reception side demands and expects the last transmission packet M which is supposed to be subjected to packet combination. Consequently, a combination of packets different from each other (packet M and packet M+1) is performed at the reception side, which defeats the purpose of enhancing the reception level by the combination, producing a quite reverse effect. On the other hand, when an error is not detected in the decoded data of the packet M at the reception side, and accordingly, when an ACK message is transmitted to the transmission side, the transmission side may, nevertheless, recognize it erroneously as a NACK, where in such a case the transmission side retransmits the last transmission packet M. Having transmitted a reception acknowledgement signal ACK, the reception side demands and expects the next transmission packet M+1. Due to the retransmission, though the reception side has already acquired data decoded with no error detected thereon, the reception side ends up decoding the retransmitted data which is identical to the already-acquired data. As its result, a problem of a significant decrease in throughput arises.
As one of prevention measures against such an erroneous combination, it is conceivable to make a judgment at the reception side as to whether the received signal is the demand one or not, where the judgment is made based on control data affixed to the packet. However, it is not possible to recognize the control data until decoding processing is performed at the reception side, causing a problem of increased processing load and of a longer processing delay.